Catalytic oxidative dehydrogenation process and catalyst

ABSTRACT

A process for the production of a mono-olefin from a gaseous paraffinic hydrocarbon having at least two carbon atoms or mixtures thereof comprising reacting said hydrocarbons and molecular oxygen in the presence of a platinum catalyst. The catalyst consists essentially of platinum modified with Sn or Cu and supported on a ceramic monolith.

This application is a Continuation of application Ser. No. 08/589,387filed Jan. 22, 1996 and now abandoned.

This invention was made with government support under grant number CTS9311295 awarded by the National Science Foundation. The government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to oxidation/dehydrogenation catalysts and aprocess for the dehydrogenation of dehydrogenatable hydrocarbons in thepresence of the oxidation/dehydrogenation catalysts and anoxygen-containing gas.

The dehydrogenation of hydrocarbons is an important commercial process.This is because of the great demand for dehydrogenated hydrocarbons asfeedstocks for industrial processes. For example, dehydrogenatedhydrocarbons are utilized in the manufacture of various products such asdetergents, high octane gasolines, and pharmaceutical products amongothers. Plastics and synthetic rubbers are other products which may beproduced through use of dehydrogenated hydrocarbons. One example of aspecific dehydrogenation process is dehydrogenating isobutane to produceisobutene which may be etherified to produce gasoline octane improvers,polymerized to provide adhesive tackifying agents, viscosity-indexactivities and plastic anti-oxidants.

2. Related Art

Various reticulated ceramic structures are described in the art: U.S.Pat. No. 4,251,239 discloses fluted filter of porous ceramic havingincreased surface area; U.S. Pat. No. 4,568,595 discloses reticulatedceramic foams with a surface having a ceramic sintered coating closingoff the cells; U.S. Pat. No. 3,900,646 discloses ceramic foam with anickel coating followed by platinum deposited in a vapor process; U.S.Pat. No. 3,957,685 discloses nickel or palladium coated on a negativeimage ceramic metal/ceramic or metal foam; U.S. Pat. No. 3,998,758discloses ceramic foam with nickel, cobalt or copper deposited in twolayers with the second layer reinforced with aluminum, magnesium orzinc; U.S. Pat. Nos. 4,810,685 and 4,863,712 disclose negative imagereticulated foam coated with active material, such as, cobalt, nickel ormolybdenum coating; U.S. Pat. No. 4,308,233 discloses a reticulatedceramic foam having an activated alumina coating and a noble metalcoating useful as an exhaust gas catalyst; U.S. Pat. No. 4,253,302discloses a foamed ceramic containing platinum/rhodium catalyst forexhaust gas catalyst; and U.S. Pat. No. 4,088,607 discloses a ceramicfoam having an active aluminum oxide layer coated by a noble metalcontaining composition such as zinc oxide, platinum and palladium.

The supports employed in the present invention are generally of the typedisclosed in U.S. Pat. No. 4,810,685 using the appropriate material forthe matrix and are generally referred to in the art and herein as“monoliths”.

The monoliths with various catalytic materials deposited thereon havealso been employed for the production of synthesis gas (PCT WO 90/06279)and nitric acid (U.S. Pat. No. 5,217,939).

U.S. Pat. No. 4,940,826 (Freide, et al) discloses the oxidativedehydrogenation of gaseous paraffinic hydrocarbons having at least twocarbon atoms or a mixture thereof by contacting the hydrocarbon withmolecular oxygen containing gas over a supported platinum catalyst wherethe support is alumina such as gamma alumina spheres and monoliths suchas cordierite or mullite. The desired products are the correspondingolefins.

Various modifiers are disclosed for the monolith/noble metal. Canadianpatent 2,004,219 lists Group IV elements as coating materials formonoliths and U.S. Pat. No. 4,927,857 discloses a platinum/monolithpartial oxidation catalyst supplemented with copper used in conjunctionwith a steam reforming process. Neither of these references suggests theuse of modified platinum/monolith catalyst in oxidativedehydrogenations.

SUMMARY OF THE INVENTION

Briefly the present invention is a process for the production of amono-olefin from a gaseous paraffinic hydrocarbon having at least twocarbon atoms or mixtures thereof comprising reacting said hydrocarbonsand molecular oxygen in the presence of a platinum catalyst modifiedwith Sn or Cu, preferably in the substantial absence of Pd and Rh on amonolith support. The catalysts consist essentially of platinum modifiedwith Sn or Cu on a ceramic monolith support, preferably alumina orzirconia monolith support.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows ethane conversion as a function of the ethane:oxygen ratiofor Sn and Cu modified Pt monolith catalyst compared to Pt alone.

FIG. 2 shows ethylene selectivity as a function of the ethane:oxygenratio for Sn and Cu modified Pt monolith catalyst compared to Pt alone.

FIG. 3 shows ethylene yield as a function of the ethane:oxygen ratio forSn and Cu modified Pt monolith catalyst compared to Pt alone.

FIG. 4 shows CO selectivity as a function of the ethane:oxygen ratio forSn and Cu modified Pt monolith catalyst compared to Pt alone.

FIG. 5 shows CO₂ selectivity as a function of the ethane:oxygen ratiofor Sn and Cu modified Pt monolith catalyst compared to Pt alone.

FIG. 6 shows H₂ selectivity as a function of the ethane:oxygen ratio forSn and Cu modified Pt monolith catalyst compared to Pt alone.

FIG. 7 shows H₂O selectivity as a function of the ethane:oxygen ratiofor Sn and Cu modified Pt monolith catalyst compared to Pt alone.

FIG. 8 plots the conversion of ethane and ethylene selectivity as afunction of the ratio of Sn:Pt.

FIG. 9 illustrates the effect of feed preheating on ethane conversion,ethylene selectivity and ethylene yield.

FIG. 10 shows n-butane conversion as a function of the butane:oxygenratio for Sn and Cu modified Pt monolith.

FIG. 11 shows i-butane conversion as a function of the i-butane:oxygenratio for Sn and Cu modified Pt monolith catalyst compared to Pt alone.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The composition of the ceramic support can be any oxide or combinationof oxides that is stable at the high temperatures of operation, near1000° C. The support material should have a low thermal expansioncoefficient. The components of the oxide support should not phaseseparate at high temperatures since this may lead to loss of integrity.Components of the oxide support should not become volatile at the highreaction temperatures. Suitable oxide supports include the oxides of Al(α-Al₂O₃), Zr, Ca, Mg, Hf, and Ti. Combinations of these can be producedto tailor the heat expansion coefficient to match the expansioncoefficient of the reactor housing.

The structure and composition of the support material is of greatimportance. The support structure affects the flow patterns through thecatalyst which in turn affects the transport to and from the catalystsurface and thus the effectiveness of the catalyst. The supportstructure should be macroporous with 30 to 80 pores per linear inch. Thepores should yield a tortuous path for the reactants and products suchas is found in foam ceramics. Straight channel extruded ceramic or metalmonoliths yield suitable flow dynamics only if the pore size is verysmall with >80 pores per linear inch.

The preferred catalyst of the present invention consists essentially ofplatinum modified with Sn or Cu (a mixture of Sn and Cu may be used)supported on a ceramic foam monolith, preferably on zirconia orα-alumina. The platinum should be deposited on the surface of theceramic to a loading of 0.2 to 90 wt. %, preferably 2 to 10 wt. %, andmore preferably in the absence or substantial absence of palladium,rhodium, and gold. It has been found that palladium causes the catalystto coke up and deactivate very quickly and thus should be excluded inany amount that is detrimental to the effectiveness of the catalyst.Though rhodium does not lead to catalyst deactivation the productdistribution is less favorable.

Preferably the Pt and modifying Sn or Cu is supported on an α-alumina orzirconia ceramic foam monolith with 30 to 80 pores per linear inch, 50to 90% void fraction, created in such a way to yield a tortuous path forreactants. The Pt and modifiers may be supported on a ceramic foammonolith comprised of any combination of α-alumina, zirconia, titania,magnesia, calcium oxide, or hafnium oxide such that the support isstable up to 1100° C. and does not undergo detrimental phase separationthat leads to loss in catalyst integrity.

In addition to Sn and Cu, several other metals were evaluated asmodifiers. Pt/Ag exhibited comparable conversion and C₂H₄ selectivity toPt alone. Experiments using Ag were identical to those described belowbut experiments were less extensive for poor catalysts (Pt/Mg, Pt/Ce,Pt/Ni, Pt/La, Pt/Co). The addition of the other metals lowered bothconversion and olefin selectivity in the order of Sn>Cu>Ptalone>Ag>Mg>Ce>Ni>La>Co as demonstrated with ethane. With lower C₂H₄selectivity, syngas (CO+H₂) formation became predominant. Pt/Au couldnot be ignited with C₂H₆+O₂. NH₃ and O₂ were used for light-off of thePt/Au catalyst, however, the catalyst extinguished quickly when C₂H₆ wasintroduced in spite of the presence of NH₃. The results on the catalystscontaining the various metals were summarized in Table I.

TABLE I Comparison of Metals Atomic ratio Reaction Conv. of S_(C2)H₄S_(COx) Y_(C2)H₄ Max. Y_(C2)H₄ Catalyst (Metal:Pt) temp ° C. C₂H₆ % % %% (at C₂H₆:O₂) Pt 0 920 69.7 64.9 26.9 45.3 52.7 (1.5) Pt/Sn 1 912 71.568.2 24.1 48.8 55.3 (1.5) Pt/Sn 3 905 72.8 68.0 24.4 49.5 55.4 (1.5)Pt/SN 7 920 75.7 69.0 21.9 52.3 57.4 (1.7) Pt/Cu 1 928 74.4 68.1 23.850.7 55.0 (1.7) Pt/Cu 3 extinguished in C₂H₆ + O₂ Pt/Ag 1 62.6 64.3 26.440.2 51.6 (1.7) Pt/Mg 3 943 65.1 60.6 33.6 39.5 43.4 (1.7) Pt/Ce 3 90560.2 49.7 47.7 29.9 31.2 (1.7) Pt/La 3 905 56.0 41.7 56.0 23.4 24.8(1.7) Pt/Ni 1 905 58.7 46.3 50.4 27.2 29.3 (1.7) Pt/Co 1 873 50.8 26.871.4 13.1 15.3 (137) Pt/Au 1 extinguished in C₂H₆ + O₂ Note. Allconversions, selectivities, and temperatures at C₂H₆:O₂ = 1.9 and 5 slpmwithout preheat. Pt loadings of all catalysts are 2 wt %.

The paraffins which are suitable for the present process are generallythose that can be vaporized at temperatures in the range of 25 to 400°C. at pressures of 0.1 to 5 atm. These are generally C₂ to C₂₀ carbonatom alkanes either alone or in mixtures, preferably having two to eightcarbon atoms. Suitable alkanes include ethane, propane, n-butaneisobutane, n-pentane, isoamylenes, n-hexane, isohexanes, n-heptane,isoheptane, octane and isooctanes. Since a preferred embodiment includesa preheating of the feed to the reaction zone, the necessity to heat analkane feed above ambient temperature to obtain a vaporous feed is not anegative consideration.

The feed may include both linear and branched alkanes. It has beenobserved in a fuel rich regime for the oxidative dehydrogenation ofn-butane that the oxygen is completely consumed, whereas for theisobutane oxidations it is not. This oxygen breakthrough suggests a ratelimiting step for isobutane. It is a proposed theory that the rates ofthese reactions should be related to the strengths of C—H bonds thatmust be broken. Thus, it may be desirable to preheat those feeds whichare determined to have relatively strong C—H bonds to increase the rateof the initiation step. The feeds may be preheated to temperatures inthe range of 0 to 500° C., preferably 25 to 400° C.

The present invention discloses the catalytic oxidative dehydrogenationof hydrocarbons. Mixtures of hydrocarbons and oxygen are flammablebetween given compositions. The feed compositions cited in thisinvention are outside the flammability limits for the citedhydrocarbons. In all cases, the feed compositions are on the fuel-richside of the upper flammability limit. The compositions range from 2 to16 times the stoichiometric fuel to oxygen ratios for combustion to CO₂and H₂O. Some molar ratios are set out below in Table II.

TABLE II Operable Fuel: Preferred Fuel: oxygen molar oxygen molar Fuelratio ratio Ethane 0.8-2.5 1.5-2.0 Propane 0.5-1.5 0.8-1.3 n-Butane0.45-1.0  0.6-0.8 i-Butane 0.45-2.25 1.4-2.1

As the diluent is reduced and as the reactants are preheated, theflammability limits widen, but it is under these conditions that higherfuel to oxygen ratios (farther from the flammable range) are preferred.This preference is based on catalyst performance with the extra measureof safety an added benefit.

Under the conditions of the present process, olefin cracking, COdisproportionation and reverse steam reforming of carbon can occur, andmay lead to coke formation. It has been found by varying the catalystcontact time, the amount of time allowed for these secondary reactionscan be controlled. At higher flow rates the olefin products spend lesstime in contact with the catalyst and higher olefin selectivities andless coking are observed.

The present invention discloses the catalytic oxidative dehydrogenationof hydrocarbons in an autothermal reactor at millisecond contact time.High yields of mono-olefins are obtained with a catalyst contact timeranging from 0.1 to 20 milliseconds when using a ceramic foam monolithof 50 to 90% porosity and 0.2 to 1 cm in depth. Under operatingconditions, this corresponds to GHSV of 60,000 to 3,000,000 hr⁻¹.

The flow rates are in the range of 60,000-10,000,000 hr⁻¹ GHSV,preferably in the range of 300,000 up to 3,000,000 hr⁻¹ GHSV may beused.

Under the conditions of the present process it can be determined thatseveral reactions may occur namely (1) complete combustion (stronglyexothermic); (2) partial oxidation to syngas (exothermic); (3) oxidativedehydrogenation (exothermic); (4) dehydrogenation (endothermic) andcracking (endothermic).

The overall process can be carried out autothermally. The heat producedby exothermic reactions provides the heat for endothermic reactions. Theprocess does not require the addition of heat.

However, improved results are obtained when moderate amounts of heat aresupplied to the system. Preheating the feed shifts the productdistribution from the more exothermic reactions (combustion and partialoxidation) to the less exothermic (oxidative dehydrogenation) andendothermic (dehydrogenation and cracking) reactions. Since oxygen isthe limiting reactant, this shift improves the process conversion. Theselectivity is improved since the less exothermic and endothermicreactions are the desired reactions.

EXAMPLES

The reactor used in the following examples consisted of a quartz tubewith an inside diameter of 18 mm containing the catalytic monolith whichwas sealed into the tube with high temperature alumina-silica cloth thatprevented bypass of the reactant gases around the edges of the catalyst.To reduce radiation heat loss and better approximate adiabaticoperation, the catalyst was immediately preceded and followed by inertalumina extruded monolith heat shields. The outside of the tube near thereaction zone was insulated.

The Pt/M (M═Sn, Cu, Ag, Mg, Ce, La, Ni, Co, and Au) bimetallic catalystswere prepared as follows: First, Pt was added to α-Al₂O₃ foam monoliths(17 mm diameter×10 mm long, 45 pores per inch (ppi) by impregnation withaqueous solutions of H₂PtCl₆. The samples were dried in vacuum at roomtemperature, followed by calcination at 100° C. for 0.5 hr. and at 350°for 2 hrs. in oxygen. The second metal was then added by impregnationwith aqueous solutions of corresponding metal salts: SnCl₂, Cu(NO₃)₂,AgNO₃, Mg(NO₃)₂, Ce(NO₃)₃, La(NO₃)₃, Ni(OOCCH₃)₂, Co(OOCCH₃)₂, andAuCl₃. The Pt/M monoliths were then dried in vacuum at room temperature,calcined at 100° C. for 0.5 hr and at 700° C. for 1.5 hrs. in oxygen,and then reduced at 700° C. for 1.5 hr. in hydrogen. Pt loadings of allsamples were either 2 or 5 wt %. The other metal loadings are summarizedin Table 1.

The catalysts are prepared by depositing Pt, a mixture of components orcomponents sequentially on commercially available ceramic foammonoliths. The foam monoliths, available from Hi-Tech Ceramics, Inc.,are composed of either α-Al₂O₃ or ZrO₂ with 30, 45 or 80 pores perlinear inch (ppi). It is important to note that these catalysts are notmicroporous structures. The monoliths are not wash-coated and areestimated to have a surface area of less than 70 m²/g. Suitablecatalysts contain 0.2 to 20 wt % Pt and tin in an atomic ratio of Pt of0.5 to 7:1 or copper in an atomic ratio to Pt of 0.5 to <3:1.

Gas flow into the reactor was controlled by mass flow controllers whichhad an accuracy of ±0.1 slpm for all gases. The feed flow rates rangedfrom 5 slpm total flow, corresponding to 37 cm/s superficial velocity(i.e. the velocity of the feed gases upstream from the catalyst,approximately 250 cm/s in the monolith at reaction conditions) at roomtemperature and atmosphere pressure. For ethane oxidation theethane:oxygen ratio was varied from 1.5 to 2.1 at a fixed nitrogendilution (30%). For butane oxidation, the butane:oxygen ratio waschanged from 0.8 to 1.4 at 50% nitrogen. In all runs, the reactorpressure was maintained at 1.4 atm. The runs were carried out with O₂ asthe oxidant. N₂ was typically added at a percent of the feed as aninternal GC calibration standard. The reaction temperature was ≈1000° C.and contact times were from 0.2 to 40 msec. Product gases were fedthrough heated stainless steel lines to an automated gas chromatograph.Shutdown of the reactor was accomplished by turning off oxygen beforealkane.

The product gases were analyzed by a gas chromatograph equipped with asingle Hayesep DB packed column. For quantitative determination ofconcentrations, standards were used for all species except for H₂O,which was obtained most reliably from an oxygen atom balance. Nitrogenwas used as an internal GC calibration standard. The selectivity datashown was calculated on a carbon atom or a hydrogen atom basis, asdescribed below.

To convert the product gas concentrations to molar quantities for agiven feed basis, the mole number change due to the chemical reactionswas calculated using the measured N₂ concentration. Since N₂ is an inertin this system, the ratio of product gas to feed gas moles was inverselyproportional to the ratio of product gas N₂ concentration to feed gas N₂concentration. Individual species concentrations were measured with areproducibility estimated to be ±2%.

Temperatures were monitored using thermocouples inserted from the rearof the quartz tube in one of the center channels of the inert monolithimmediately after the catalytic monolith. The reactor was operated at asteady state temperature which is a function of the heat generated bythe exothermic and endothermic reactions and the heat losses from thereactor.

Although the process in steady state is autothermal with feed gases atroom temperature, heat was supplied initially to ignite the reaction. Amixture of hydrocarbon and air near the stoichiometric composition forproduction of synthesis gas was fed to the reactor, and the reactantswere heated to the heterogeneous ignition temperature (≈230° C. for C₂to C₄ hydrocarbons). After light-off, the external heat source wasremoved (unless feed preheating is indicated), the reaction parameterswere adjusted to the desired conditions, and steady state wasestablished (≈10 min) before analysis. For situations where the catalystwas not ignited with a mixture of alkane and oxygen, e.g. Ag as amodifier, a NH₃/O₂ was used for light-off and NH₃ was then graduallyexchanged for the alkane. Data shown were reproducible for time periodsof at least several hours and on several catalyst samples.

For C₂H₆ oxidation, the major products over all catalysts were C₂H₄, CO,CO₂, CH₄, H₂, and H₂O. Traces of C₂H₂, C₃H₆, C₃H₈, and C₄H₈ wereobserved, usually with selectivities <2%. The conversions of oxygen werealways above 97%, so reactions always go to completion.

Example 1 Ethane Pt, Pt/Sn and Pt/Cu Catalysts

FIGS. 1, 2, and 3 show the C₂H₆ conversion, C₂H₄ selectivity, and C₂H₄yield for oxidative dehydrogenation of ethane over Pt, Pt/Sn(Sn:Pt=7:1), and Pt/Cu (Cu:Pt=1:1) as a function of the feed composition(2.0 is the ethylene stoichiometric ratio). With increasing feedcomposition, the conversion decreased while the selectivity increasedover the three catalysts. The addition of Sn significantly enhanced boththe conversion (by ≈7%) and the selectivity (by ≈5%), which produced thehighest C₂H₄ yield of 57% at 25° C. feed in this study. The Pt/Cu alsoshowed higher conversion and higher selectivity than Pt, the maximumyield being 55%. As shown in FIGS. 4, 5, 6 and 7, both Pt/Sn and Pt/Cushowed 5≈9% lower CO selectivity and 1≈₂% higher CO₂ selectivity thanPt. Among minor products, more C₂H₂ and C₄H₈ were formed on both Pt/Snand Pt/Cu than on Pt. The addition of Sn or Cu inhibited CO productionand promoted the formation of olefins and acetylene without significantchange in CH₄ selectivities.

The reaction temperatures decreased from 1000 to 900° C. as the C₂H₆:O₂ratio increased from 1.5 to 2.1 and temperatures were same to within±20° C. on these three catalysts.

No deactivation or volatilization of the catalysts were observed forseveral hours. No significant coke formation on the catalysts wasobserved.

Example 2 Ethane Loadings of Pt, Sn, and Cu

FIG. 8 shows plots of C₂H₆ conversion and C₂H₄ selectivity as functionsof Sn:Pt ratio at a feed near the oxidative dehydrogenationstoichiometry (C₂H₆:O₂=1.9). The conversion increased with increasedSn:Pt ratio. On the other hand, the addition of a small amount of Sn(Sn:Pt=1:1) enhanced the selectivity significantly and the furtheraddition led to a slight increase in the selectivity.

Pt/Cu (Cu:Pt=1:1) showed comparable results to Pt:Sn, as describedabove. However, Pt/Cu (Cu:Pt=3:1) could not be ignited in the mixture ofC₂H₆ and O₂. A NH₃:O₂ mixture was used for ignition, but the catalystextinguished upon exchange of NH₃ for C₂H₆,

A sample of 5 wt % Pt was nearly identical to 2 wt % Pt, although theC₂H₆ conversion was 1% lower with 5 wt % loading. The addition of Sn to5 wt % Pt also enhanced both the conversion and C₂H₄ selectivity. The 5wt % Pt/Sn (Sn:Pt=1:1) exhibited comparable results (1% higherconversion and 1% lower selectivity to 2 wt % Pt/Sn (Sn:Pt=1:1). Thisfact confirms that Sn acts as a promoter for ethane oxidation,regardless of Pt loadings. Neither 5 wt % Pt/Cu (Cu:Pt=1:1) the 2 wt %Pt/Cu (Cu:Pt=3:1) worked autothermally.

Example 3 Preheat

FIG. 9 shows the effect of preheat on the conversion, selectivity, andyield over Pt/Sn (7:1) catalyst at C₂H₆:O₂=1.9. Preheat of reactiongases up to 400° C. increased the conversion from 77 to 89% anddecreased the selectivity from 69 to 65%, which led to an increase inyield from 53 to 58%.

Example 4 n-Butane

Oxidative dehydrogenation of n-butane was examined over Pt, Pt/Sn(Sn:Pt=3), and Pt/Cu (Cu:Pt=1). Both Pt/Sn and Pt/Cu showed much higherC₄H₁₀ conversion (by ≈16%) than Pt as a function of feed composition(FIG. 10). On the three catalysts, the selectivities to C₂H₄ and CO_(x)decreased and selectivity to C₃H₆ increased with increasing C₄H₁₀:O₂ratio. The C₄H₈ selectivity was only 3-5% and increased slightly withincreasing C₄H₁₀:O₂ ratio. The C₂H₄ selectivity from n-C₄H₁₀ was muchhigher on Pt/Sn and Pt/Cu than on Pt, while the C₃H₆ selectivity wasmuch lower on Pt/Sn and Pt/Cu than on Pt.

Example 5 i-Butane

Oxidation of i-butane was similar to n-butane. Both Pt/Sn (Sn:Pt=3) andPt/Cu (Cu:Pt=1) showed much higher conversion (by 15-25%) than Pt (FIG.11). With i—C₄H₁₀ the dominant olefins are i-C₄H₈ (≈30%) and C₃H₆(≈30%). On all three catalysts, selectivities to C₂H₄ decreased andselectivities to C₃H₆ and i-C₄H₈ increased with increasing C₄H₁₀:O₂ratio. As a function of conversion, Pt/Sn and Pt/Cu exhibited higherselectivities to olefins and acetylene than Pt at high conversion.

XRD

X-ray diffraction patterns were determined for Pt and Pt/Sn (Sn:Pt=1:1and 7:1) catalysts after reaction. On Pt catalyst, only peaks of Ptmetal were observed except for that of the α-Al₂O₃ support. On the otherhand, only PtSn and Pt₃Sn peaks were observed for Pt/Sn catalysts andthere were no Pt metal peaks. The PtSn:Pt₃Sn ratio was higher for Pt:Sn(1:7) than for Pt:Sn (1:1). These results clearly indicate that Ptexists in the forms of only Pt₃Sn and PtSn alloys on support for Pt/Sncatalyst.

The addition of Sn or Cu to Pt-monolith enhanced alkane conversion andolefin selectivities and suppressed CO_(x) formation for the oxidativedehydrogenation reactions. Since Pt exists in the forms of only PtSn andPt₃Sn alloys on Pt/Sn catalyst, it is speculated that PtSn and Pt₃Snalloys are the active sites and are more selective to C₂H₄ formationthat Pt.

1. A catalyst composition consisting essentially of platinum alloyedwith tin deposited on a monolith in an atomic ratio of Sn:Pt of 0.5:1 to7:1, wherein there is no unalloyed platinum as determined by x-raydiffraction.
 2. The catalyst composition according to claim 1 whereinthe monolith comprises ceramic.
 3. The catalyst composition according toclaim 1 wherein the monolith comprises oxides of Al, Zr, Ca, Mg, Hf, Ti,or mixtures thereof.
 4. The catalyst composition according to claim 1wherein the catalyst was prepared by codepositing Pt and Sn on themonolith.
 5. The catalyst composition according to claim 1 wherein thecatalyst was prepared by sequentially depositing Pt and Sn on themonolith.
 6. The catalyst composition according to claim 1 wherein themonolith has a surface area of less than 70 m²/g and has 30 to 80 poresper linear inch.
 7. The catalyst composition according to claim 1consisting essentially of Pt alloyed with Sn deposited over a zirconiamonolith having 30 to 80 pores per linear inch and less than 70 m²/gsurface area.
 8. The catalyst composition according to claim 1consisting essentially of Pt alloyed with Sn deposited over an aluminamonolith having 30 to 80 pores per linear inch and less than 70 m²/gsurface area.
 9. The catalyst composition according to claim 1 whereinthe monolith is a ceramic foam monolith.
 10. The catalyst compositionaccording to claim 1 wherein platinum is present in an amount of 0.2 wt% to 10 wt %, based on the total weight of the ceramic monolith andplatinum catalyst.